The most important finding of this study was observing significantly different cartilage thickness measurements at the distal lateral femoral condyle, in patients who underwent three-point cartilage thickness measurement during kinematically aligned robot-assisted TKA. However, the mean of the three-point cartilage thickness measurements was found to be similar to the measurement of the originally described kinematically aligned TKA technique that was accepted as 2 mm. Our radiographic outcomes demonstrated that patient-specific kinematically aligned TKA by cartilage thickness measurement did not significantly change mLDFA and JLOA, but significantly corrected HKA, mMPTA, and JLCA. Preliminary clinical results showed a significant improvement in all scores as expected. Our null hypothesis can be partially accepted that every patient has a different cartilage thickness, and kinematically aligned TKA by patient-specific cartilage measurement significantly corrected the deformity without changing patients’ JLOA.
Since its first description, kinematically aligned TKA has gained interest and popularity by years with the help of the development in computer-assisted patient-specific instrumentation and robot-assisted surgery. To date, many prospective randomized controlled clinical studies that compared mechanically and kinematically aligned TKA reported either similar outcomes or superior results in favor of kinematically aligned TKA [14–23]. The main advantage of kinematically aligned TKA is restoring the patient’s original anatomy and resurfacing the bones by making the bone cuts according to the patients’ native anatomy and setting the bone cuts according to the thickness of the implant. In addition, prevention of the excessive release of medial collateral ligament (MCL) and restoring the constitutional varus also affect the kinematic and the function of the knee joint. In their study on 250 healthy subjects, Bellemans et al. found the mean HKA as 1.87-degree varus in males and 0.79-degree varus in females [9]. In another study by Almaawi et al., 4884 lower limb computed tomography (CT) were scanned and the authors reported that only 5.5% of the patients had a HKA of 0 degree [24]. In our study, we performed kinematic alignment in all cases and the mean planned HKA was 1 ± 0.9 degree. Nevertheless, we found a mean 2.3 ± 3 degrees at postoperative standing full-length radiographs. This difference can be explained by the lying robotic and standing radiographic measurements. We also noticed this difference in the studies of Bellemans et al. and Almaawi et al. Despite Bellemans et al.’s study, Almaawi et al. reported a mean HKA of 0.1-degree varus in their CT scan study of a large population that also performed in lying position [11, 24]. This issue should be investigated by further population studies in larger cohorts to define the exact value of HKA angle in normal pre-arthritic knee.
Joint-line orientation is still in debate and in the center of the discussion between kinematically and mechanically aligned TKA. There are limited long-term studies in the literature evaluating the JLOA and its effect on functional outcomes. In a recent study by D’amato et al., the authors mentioned that restoration of the JLOA did not affect the clinical outcomes in the postoperative 1-year [25]. In another recent study, Celek et al. reported that restoring the tibial joint line (joint-line orientation angle) parallel to the floor had no effect on outcomes [26]. D’Amato et al. and Celek et al. both observed that patients with medially opened JLOA had better outcomes than their laterally opened opposites [25, 26]. A medially opened JLOA also means a residual varus position of the tibia that can be observed in patients with constitutional varus. Shin et al. also remarked on this issue and reported better functional and joint awareness scores in their TKA technique preserving the constitutional varus [27]. In our study, we found no significant difference between preoperative and postoperative JLOA measurements, and this finding demonstrated that patients’ native JLOA was preserved. Interestingly, we observed a high variation in JLOA measurements that showed the diversity of the anatomy of the patients. The idea of kinematically aligned TKA is based on this concept in which every patient is different and also patient’s every leg is different, so rather than adjusting the bone cut perpendicular to the 0-degree mechanical axis, the patient’s native anatomy should be preserved. The reason for observing similar mLDFA measurements can be explained by making the bone cuts in concordance with the distal femoral cartilage wear. In addition, the coronal degree of tibial bone cuts that is set in relation to the femur also does not change the patients’ JLOA, which is also described as a tibial joint-line orientation angle.
In this study, as expected, we observed significant increases in the mean KOOS scores and Oxford Knee scores after kinematically aligned robot-assisted TKA. The Forgotten Joint Score is an important outcome measurement especially for kinematically aligned TKA due to its philosophy of preserving the joint kinematic and the patient’s native anatomy. Elbuluk et al. compared mechanical and kinematic alignment in their study and observed significantly better FJS at postoperative 1-year and 2-year [28]. However, Mc Ewen et al. did not observe a significant difference between computer-assisted mechanic and kinematic alignment in terms of functional scores and FJS [29]. In our study, we have no chance to compare the functional scores and FJS of patients with another group. We detected a mean 67.6 point of FJS in the present study. Singh et al. reported a relatively small FJS with a mean of 38.2 in postoperative 1-year [30]. In another study of Singh et al., authors observed a mean FJS of 42.6 and threshold value between 33.3 and 77.1 for detecting patient acceptable symptom state in primary TKA [31]. Zuiderbaan et al. and Peersman et al. reported a mean 59.3 and 54.8 FJS, respectively, at postoperative 1-year [32, 33]. Our joint awareness outcome by FJS is consistent with the current literature. Nevertheless, there are limited studies in the literature evaluating FJS in kinematically aligned robot-assisted TKA.
The retrospective design of the study is the major limitation of our study. However, we evaluated a prospectively followed patient population operated by a single surgeon in a single center. Lack of control group is another limitation of this study. Nevertheless, we aimed to only present the preliminary outcomes of kinematically aligned robot-assisted TKA by patient-specific cartilage thickness measurement. The main strength of our study is being the first study that assesses the cartilage thickness of the knee joint intraoperatively for the application of kinematic alignment. In addition, our study is the largest case series in the literature reporting the outcomes of robot-assisted TKA performed by NAVIO Surgical System.